The federal government reports that it is doing a decent job eliminating the gender gap within government — although, despite all the bureaucracy and job descriptions and computerized hiring processes, there is still an 11 percent difference between men's and women's salaries there. That gap isn't only attributable to it being an average, either.

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All but 7 cents of the gap can be accounted for by differences in measurable factors, such as differences in education levels and the type of jobs men and women had, the report said. The gap narrowed the more men and women shared characteristics, including the jobs held, levels of experience and education.

The GAO said factors such as work experience outside government and discrimination may account for some or all of the remaining gap.

Another way to say that would be that one-third of the gap is attributable to reasonable differences between men and women (which is what conservatives mean when they say the wage gap is due to "choices") and two-thirds of it is attributable to unreasonable factors, like discrimination.

When U.S. women's annual incomes were averaged, it came out to $34,400 compared with $44,300 for men. Over four decades that average $9,900 gap can mean anywhere between $500,000 and $1 million in lost income for an individual woman.

Women in some highly paid professions—such as law—stand to lose $2 million or more in full-time careers.

Unsurprisingly, women of color are affected the worst.

The study also took a look at demographics, finding that women earn 78 cents for every dollar earned by a white male worker, the highest-paid group in the work force.

The income disparity widens for minority women, with African American women earning 67 cents on the white male dollar and Hispanic women getting only 58 cents.